In recent years, glass material has found a variety of applications and has given rise to a need for surface polishing of the material in some cases. For example, during production of optical lenses and glass substrates for optical lenses, the surface of glass material must be polished to a very high degree of surface fineness so as to provide a mirror surface. Particularly, flat and defect-free surfaces with minimal surface roughness are required for glass substrates for optical and magnetic disks, those for liquid crystal displays, such as thin-film transistor (TFT) type and twisted nematic (TN) type LCDs, those for color filters of liquid crystal television displays and those for LSI photomasks. This brings about a demand for a surface polishing technique of higher precision.
High heat resistance is demanded of glass substrates for liquid crystal displays, since such substrates undergo high-temperature post-heat-treatment. In addition, substrates have become thinner and thinner in the trend toward weight reduction. Regarding a magnetic disk glass substrate, there are also demands to reduce the thickness of the substrate keeping with the trend in weight reduction and high mechanical strength, particularly rigidity, so as to withstand, for example, deformation of the disk during high-speed rotation. The levels of these demands have become higher year by year.
In order to meet the aforementioned demands for reduced thickness and mechanical strength, improvements have been made to the chemical composition of glass and to the method for producing glass, providing use of a glass substrate predominantly containing aluminosilicate as a substrate for liquid crystal displays or magnetic disks. Regarding a magnetic disk glass substrate, there have conventionally been developed glass-ceramic substrates predominantly containing lithium silicate or quartz crystals as a major component. The glass of these substrates has considerably poor processability, and therefore, when a conventional abrasive is used, processing speed is low to thereby deteriorate productivity. Thus, there are demands for a large polishing rate and high-precision surface polishing performance.
Regarding conventional abrasives employed for surface polishing of a glass substrate, there has been employed an abrasive predominantly comprising rare earth oxide, inter alia cerium oxide, since cerium oxide exhibits a polishing rate several times that of iron oxide, zirconium oxide or silicon dioxide. During use of such an abrasive, abrasive grains are generally dispersed in liquid, such as water. When a conventional cerium-oxide-containing abrasive is used for polishing the aforementioned glass substrate of high hardness, there arises a problem of a poor polishing rate.
Although the polishing mechanism of a cerium-oxide-containing abrasive has not been fully elucidated, it has been phenomenologically confirmed that the polishing proceeds on the basis of synergistic effect of chemical action of cerium oxide on glass and mechanical action attributable to the hardness of cerium oxide particles themselves. However, since a glass substrate predominantly comprising aluminosilicate and a glass-ceramic substrate predominantly comprising lithium silicate have excellent resistance to chemicals, the chemical action of the abrasive is not fully attained. In addition, collapsing of abrasive grains readily occurs, because of the high hardness of these glass substrates (to be processed), resulting in failure to maintain sufficient mechanical action on glass and consequently in deterioration of the processing rate.
In order to maintain the mechanical action for a long period of time, addition of powdered particles of, e.g., alumina or zirconia having higher hardness than a substrate to be processed, to an abrasive composition may be a conceivable approach. However, when this approach is followed, the relative concentration of cerium oxide decreases, resulting in poor chemical action. In addition, powdered particles of high hardness adversely impart defects, such as pits and scratches, to the glass surface (of the substrate to be processed).
The present invention has been accomplished so as to solve the aforementioned problem involved in conventional techniques. Thus, an object of the invention is to provide a process for producing a cerium-containing abrasive that maintains for a long period of time an initial polishing rate relative to a glass material which is hard and is not readily polished at a large polishing rate, and imparts no surface defects, such as pits and scratches, to a material to be polished, such as glass, to thereby provide a polished surface of high quality. Another object of the invention is to provide a cerium-containing abrasive produced through the process.